This study attempts to elucidate the relationship between the structural and immunological properties of substructures of a complex protein molecule, Bovine Serum Albumin (BSA), and provide information on how the immune system recognizes and responds to such molecule. Highly purified fragments of BSA have been isolated and have been shown to be poorly immunogenic but highly immunomodulating. The individual antigenic fragments carry unique determinants not shared with other fragments. A single antigenic fragment, however, has the capacity to suppress immuno response to the entire BSA molecule which seem to indicate that the regulation of immunosuppression is not dependent on serological determinants but on some other presently unknown property of the fragment. To further characterize distinct epitopes monoclonal antibodies to defined antigenic fragments using hybridoma technology will be used. These antibodies will be a source for investigating cross-reactivity of idiotypes. We shall also attempt to clarify the relationship between idiotypic and antigenic effects and specify at which cell level epitopes or idiotypes influence immune response to BSA molecule.